The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as, but not limited to, carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions, typically disposed on catalyst supports or substrates, are provided in an engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
A technology that has been developed to reduce the levels of NOx emissions in lean-burn engines (ex. diesel engines) that burn fuel in excess oxygen includes a Selective Catalytic Reduction (“SCR”) device. The SCR catalyst composition applied to the SCR device may contain a zeolite and one or more base metal components such as iron (“Fe”), cobalt (“Co”), copper (“Cu”) or vanadium (“V”) which can operate efficiently to reduce NOx constituents in the exhaust gas in the presence of a reductant such as ammonia (‘NH3”). When used in conjunction with a diesel engine the reductant is commonly referred to as a Diesel Emissions Fluid or (“DEF”) and is injected at a location upstream of the SCR device during operation of the engine. For example, when the proper amount of NH3 is available in the SCR device under the proper conditions, the NH3 reacts with the NOx in the presence of the SCR catalyst to reduce the NOx emissions to, for example, nitrogen. Reductant storage systems are provided to supply any needed reductant to the SCR device.
NOx sensors are typically placed both upstream and downstream of the SCR device to determine the efficiency of NOx conversion by the SCR catalyst composition. It is important that the SCR device maintains a high level of NOx conversion in order to maintain compliance with emission regulations. As such, when various exhaust system controllers determine, through signal communication with the NOx sensors that the SCR conversion efficiency of the SCR device may not be converting NOx at a desired level of efficiency a code, or other type of warning, signals the operator that service may be necessary. Due to the high cost of replacement, it is important that appropriate diagnostic tests be performed before the SCR device is replaced.